A. Field of the Invention
The present invention is related to the carrier phase from a digitally controlled oscillator (DCO). More particularly this invention relates to obtaining accuracy of a carrier phase significantly less than the least significant bit (LSB).
B. Problems in the Art
The advent of carrier phase detection devices, particularly in the Global Positioning System (GPS) field, adds a high impetus to develop an accurate, yet low cost, method of simulating carrier phases down to sub-millimeter carrier phase accuracy.
To achieve this accuracy, very high quality, and therefore higher cost, phase control hardware is used. Alternatively, a special high rate processing that provides phase by averaging is used. However, this latter methodology of essentially averaging between two consecutive Least Significant Bit (LSB) phases, thus simulating a phase angle between these two, not only requires significant processing overhead, but typically exhibits xe2x80x9cjitterxe2x80x9d or a xe2x80x9cwaverxe2x80x9d between the two phase angles truly present and the desired phase angle simulated by the processing.
An LSB is typically expressed as a numerator (p) and a denominator (q), wherein p and q are relatively prime, which gives a xcex94.                     Δ        =                  p          q                                    Eq        .                  xe2x80x83                ⁢        1            
and thus producing an error range of p/q, as shown in FIG. 2. Therefore writing a 1 to the carrier DCO gives a phase change of xcex94. Therefore subsequent DCO commands would give:
For example, in the above case, to simulate a phase angle of xcex94/2, one would switch between a 1 and a 0 command in a one to one ratio. If a phase angle of xcex94/4 is desired, a command of 0 is entered for a time period three times as long as a command of 1. Clearly, other angles can be xe2x80x9csimulatedxe2x80x9d by averaging, but it would take significant processing and the xe2x80x9cjitteringxe2x80x9d effect would be exhibited from this averaging method.
The present invention includes a method for determining an appropriate value to be entered into a digitally controlled oscillator to achieve better than least significant bit accuracy.
The present invention also includes a computer readable medium having computer executable instructions for causing a computer to determine an appropriate value to be entered into a digitally controlled oscillator to achieve better than least significant bit accuracy.
This is done by: choosing a desired phase angle xcex8; solving the base equation
Ap+Bq=1xe2x80x83xe2x80x83Eq.2
for A, wherein the values of p and q are relatively prime components of an LSB; and computing a command value, k, from the command equation
k=[Aint(qxcex8)]mod(q)xe2x80x83xe2x80x83Eq.3
From this k value, an appropriate command value can be entered. On most DCOs it will be the command value itself. However those skilled in the art will appreciate different requirements for different DCOs and will appreciate the relationship between the k value and the value to be entered into the DCO.